Indirect Fluid Flow Measurement

ABSTRACT

A method for ablating tissue comprising controlling a heating element using a variable phase angle control to heat an ablation fluid to a desired temperature and determining a heating percentage corresponding to a percentage of a maximum available heating power represented by a current level of power supplied to the heating element and, when the heating percentage remains below a threshold level for a predetermined period of time, indicating a flow obstruction condition of the fluid.

PRIORITY CLAIM

This application claims the priority to the U.S. Provisional ApplicationSerial No. 61/017,426, entitled “Indirect Fluid Flow Measurement” filedon Dec. 28, 2007. The specification of the above-identified applicationis incorporated herewith by reference.

FIELD OF THE INVENTION

The present invention relates to systems and methods for determining arate of fluid flow through an ablation device.

BACKGROUND

One technique for the treatment of menorrhagia relates to the ablationof uterine lining using heated fluid which is withdrawn via a returnlumen. Such systems are heavily reliant on the maintenance ofsubstantially constant circulating fluid level and flow rate within theuterus to ensure the safety, efficiency and effectiveness of thetreatment. Undesirable circulating fluid levels and flow rates mayprevent the heated fluid from reaching and properly ablating the entiresurface area of the lining of the uterus.

SUMMARY OF THE INVENTION

The present invention is directed to a method for ablating tissuecomprising controlling a heating element using a variable phase anglecontrol to heat an ablation fluid to a desired temperature anddetermining a heating percentage corresponding to a percentage of amaximum available heating power represented by a current level of powersupplied to the heating element and, when the heating percentage remainsbelow a threshold level for a predetermined period of time, indicating aflow obstruction condition of the fluid.

The present invention is further directed to a system for thermalablation comprising a heating element for heating an ablation fluid anda computing arrangement controlling power supplied to the heatingelement based on a proportional, integral and derivative (PID)algorithm, the computing arrangement monitoring power required by theheating element to sustain a desired temperature of the ablation fluidand executing a safety procedure when the power supplied to the heatingelement over a predetermined period of time drops below a thresholdlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings:

FIG. 1 shows a pulse width modulated signal according to exemplaryembodiments of the present invention.

FIG. 2 shows phase control signals according to an exemplary embodimentof the present invention.

DETAILED DESCRIPTION

The present invention may be further understood with reference to thefollowing description. The present invention relates to a system andmethod for regulating fluid flow and fluid level within a device forthermally ablating tissue, e.g., tissue lining an inner surface of ahollow organ. In particular, the present invention relates to devicesfor ablating the endometrium. However, those skilled in the art willunderstand that the present invention, and/or components thereof, may beutilized in conjunction with devices for prostate treatment (microwaveor cyroablation), irrigation systems or other devices for procedureswhich infuse heated fluids to the body.

Fluid ablation systems generally utilize one or more resistive heatingelements to warm a circulating fluid (e.g., saline) to a predetermined,substantially constant temperature. For example, such a thermal ablationsystem may heat the ablation fluid within a disposable cassette portionwhich is removably coupled to a reusable console. The heated ablationfluid may then be circulated through a hollow organ to ablate the liningthereof and returned to the system either for recirculation or disposal.One such system is described in a U.S. Patent Application entitled“Thermal Ablation System”, naming as inventors Robert J. Bouthillier,Michael P. Fusaro, Joseph M. Gordon, Stephen S. Keaney, Brian MacLean,Andrew W. Marsella, David Robson and Boris Shapeton filed on Nov. 14,2007 and assigned Ser. No. 60/987,913. The entire disclosure of thisapplication is hereby expressly incorporated by reference herein. Thoseskilled in the art will understand that, although the precisetemperature requirements for the ablation fluid may vary according tothe type of procedure being performed, these systems are generallyrequired to maintain this temperature within a narrow range for theentirety of the procedure to avoid complications such as burns where thetemperature is excessive or a failure to sufficiently ablate the targettissue where the fluid temperature falls below the desired temperaturerange. The heating element may, for example, be powered by analternating current (“AC”) power source under control of a closed loopmicroprocessor based Proportional, Integral and Derivative (“PID”)control. For example, a uterine ablation procedure requiring an ablationtemperature of 90°±3° Celsius and more preferably 90°±2°, may require anAC power of 500 watts and a constant circulating fluid flow of about200-300 ml/min.

The fluid follows a path passing a heating element which elevates thefluid temperature to the desired level, after which the fluid sweepspast thermistors, which detect the temperature of the fluid and transmitit to the PID. The fluid is then circulated through the hollow organafter which it is filtered and returned to the heating element forrecirculation to the organ in a closed loop. A disruption of flow maycause fluid to collect by the heating element where it will be heatedpersistently, potentially elevating the temperature outside theallowable range. Devices according to the present invention identifywhen this situation has arisen and prevent fluid that has beenexcessively heated from flowing back to the body.

Those skilled in the art will understand that varying amounts of energymay be required during initial heating as well as during the ablationprocess to maintain the desired temperature. The main factors affectingthe energy required to maintain a desired fluid temperature are fluidflow rate and heat loss to the patient and surroundings. Those skilledin the art will understand that the precise amount of required energy isalso affected to a lesser degree by additional factors including ambienttemperature conditions, the age and body fat index of the patient,humidity, heat absorption by the tissue, etc. Accordingly, the thermalablation system must constantly compensate for the heat loss andactivate or deactivate the heating system accordingly. In addition, thethermal ablation system must react to changes in fluid flow rate (e.g.,to obstruction) as the average amount of energy required to maintain thetemperature of a static volume of water decreases significantly. Sincethe fluid is no longer flowing to the treatment area where it losesheat, additional heating results in an increased temperature thereof.Current thermal ablation systems are designed to generate target flowlevels but blockages, tubing kinks, pump malfunctions, etc. mayfrustrate this purpose.

A device according to an exemplary embodiment of the present inventionenhances the ability of thermal ablation systems to address theseconcerns by generating an alert when such a flow occlusion is detected.The present invention employs a system measuring the average heaterpower required to maintain the desired temperature over time and, basedon this power data, determines a rate of fluid flow through the heatingchamber. Changes in the rate of flow through the heating chamber maythen be used to detect problems with flow.

According to the exemplary embodiment, a software based PID algorithmcontrols the heating of the ablation fluid to a nominal set temperature.However, those skilled in the art will understand that any suitablealgorithm, such as a PWM, an ON/OFF control, etc, may be employed tocontrol the heating of the fluid and obtain the same results describedherein. The PID algorithm adjusts power to the heating element bycalculating a delay from the zero crossing of each AC cycle half wavebefore allowing any current to flow therethrough. As the delay periodincreases, the average current driven into the heating elementdecreases. Since this type of control is inherently non-linear (i.e.,the size of the AC instantaneous voltage varies with delay), the PIDcompensates for the delay value so that the PID can linearly adjustpower from zero to full-scale, as shown with respect to FIG. 1. As theamount of heat required to bring the fluid up to the desired temperatureincreases, the on time of the phase control signal also increasesproportionally, in the direction A (i.e., the pulses grow increasinglywide while the gaps between pulses narrow). Those skilled in the artwill understand that the particular method by which the PID alters thepower does not impact operation of the system according to the presentinvention. For example, a system might maintain the size of the pulse byvarious means, such as controlling a percentage of full or half wave ACcycles in a given period of a PWM or simply controlling the on or offtimes to maintain a threshold around a setpoint temperature. In analternate embodiment, an adjustable current source could be used toregulate the produced heat.

As further detailed with respect to FIG. 2, the phase control signal maymodulate a waveform 10 or 20 in response to an on pulse 5, 15 or an offpulse received from a controller. For example, the waveform 10 ismodulated in response to the on pulse 5, wherein the timing of thereceived on pulse 5 allows one half cycle of the waveform to bemodulated. The off pulse is indicated by the termination of the waveform10. The waveform 20, on the other hand, is modulated in response to anon pulse 15, wherein the timing of the pulse 15 allows for less thanhalf of one wave cycle to be modulated. Those skilled in the art willunderstand that the phase control allows a solid state relay to turn onan adjustable amount of time after a zero crossing of the AC power wave,resulting in signals that are produced at varying stages of the AC powerwave.

When the detected fluid temperature exceeds the setpoint, the PIDalgorithm reduces heat supplied to the system and conversely, when thetemperature is below the setpoint, the PID algorithm increases heatsupplied to the system. Varying temperature requirements from the heatcontrol cause the PID to fluctuate from condition 1, when the maximumheat is generated, to condition 5 in which no heat is generated andthough any of the intermediate conditions 2-4.

Those skilled in the art will understand that, when the temperaturepersists at a value greater than the setpoint, the PID may call for anegative heating value (i.e., cooling). In systems without an activecooling arrangement, this condition is equivalent to the heat offcondition. As would be understood by those skilled in the art, systemsthat do not have active cooling are dependent on the natural heat losscharacteristics of the system to lower temperature and are thereforegenerally slower in response than systems including an active coolingsystem.

The PID algorithm keeps track of the percentage of full scale heat thesystem is currently supplying via the heater element. Since the phasecontrol is not linear in heat produced with phase delay, the PID uses alook up table to linearize the output so that heat can be linearlyapplied between zero and full scale.

The following exemplary code is one example of a suitable control logicfor the PID algorithm.

if( s_bTestFlowFault && !g_bInhibitFlowFault )     {     if( GetPcntFS() <= 0 &&     ( ( g_cTemp1Rate >=     −2 && (g_iTempC1×10 >=(HIGH_TEMP_LIMIT/10L))) ||     g_iTempC1×10 > (10+HIGH_TEMP_LIMIT/10L)))       {        if( s_ulLocalTime > (s_ulLowFlowTime +       OCCLUSION_TIMEOUT ) )           {           s_ucNextPhase =s_Ph->ucLowFlowNextPhase;           g_ucExitCode = LOW_FLOW;          break;           }        } else if( GetPcntFS( ) < 5 )     {    if( s_ulLocalTime > (s_ulLowFlowTime +     (3*OCCLUSION_TIMEOUT)))       {        s_ucNextPhase = s_Ph->ucLowFlowNextPhase ;       g_ucExitCode = LOW_FLOW;        break;        }     } else     {    s_ulLowFlowTime = s_ulLocalTime ;     } }

The above exemplary embodiment references the variables noted below.Those skilled in the art will understand that the noted values areexemplary only , specific to a standard uterine ablation procedure andthat these values may be altered according to the requirements of theprocedure to be performed.

-   -   GetPcntFS( )returns the required Percentage of full scale heat        (“% FS”) to heat the fluid to a nominal temperature and may hold        a value from −100% to 100%.    -   g_iTempC×10 is the temperature in tenths of a degree Celsius        (i.e., 90° C. is 900). This value is obtained via sensors (e.g.,        the thermistors) in the thermal ablation device, which are        connected to the PID.    -   g_iTemp1Rate corresponds to the temperature rate of change in        tenths of a degree Celsius/second.    -   HIGH_TEMP_LIMIT corresponds to the user-defined limit of the        maximum temperature of the ablation fluid. For example, an upper        limit of 90° C. would equate to a value of 900.    -   OCCLUSION_TIMEOUT is defined as 5.0 seconds.

It is to be noted that the employment of the PID algorithm mayconstitute defining threshold values as described below which, whenexceeded, indicate undesirable flow values. In reference to the above Ccode, the PID algorithm may perform the functions as noted below.

If the PID (1) calls for no heat, (2) indicates a temperature rate ofchange greater than a first threshold and (3) indicates that thetemperature is above a second threshold and that this condition haspersisted for a first predetermined time, a no flow condition may bedeclared, where the first threshold refers to rate of increase of fluidtemperature and the second threshold refers to a first maximumtemperature. Consequently, when these conditions are met, a userinterface of the thermal ablation system may notify a clinician thatfluid flow is obstructed. The notification may be carried out in any ofa variety of manners (i.e., audible message, visual message on displayscreen of thermal ablation device, etc.). Additionally, the thermalablation device may respond to the no flow declaration by automaticallysuspending the supply of heated fluid to the patient (i.e., bytemporarily sealing the delivery lumen of the thermal ablation device,etc.) or by initiating any desired safety procedures relevant to thiscondition.

Additionally, if the temperature of the fluid, as read by thethermistors, is above a higher third threshold for a secondpredetermined time period, which may or may not be substantially equalto the first predetermined time, a no flow condition may be declaredregardless of the other conditions. In this situation, the heat calledfor, temperature rate of change, etc. may be disregarded and a no flowcondition declared. The third threshold refers to a second maximumtemperature level allowable for the procedure, taking into account theallowable temperature deviation, which may vary in accordance with theprocedure being performed, as mentioned earlier. If either of the abovenoted threshold conditions persists for a predetermined period of time,typically 5.0 seconds, the no flow condition will be declared.

In a second test, when the required Percentage of FS (“% FS”) is below aparticular predetermined value continuously for a second predeterminedtime (e.g., a period of time significantly longer than the firstpredetermined time), a no flow condition may be declared. For example,the second predetermined time may typically be 15 seconds. The % FS isindicative of the PID calling for little or no heat or, alternatively,for cooling (negative values). The second test may, for example, entaila situation where a procedure has been resumed without correcting apreviously indicated flow problem. In this case, no further temperaturerise may be expected since the PID has already throttled down the % FSheat.

The above detailed code may be implemented in a thermal ablation devicein order to control and monitor the temperature and flow of fluidthrough the device. Those skilled in the art will understand that thedescribed exemplary embodiments of the present invention may be alteredwithout departing from the spirit or scope of the invention. Thus, it isto be understood that these embodiments have been described in anexemplary manner and are not intended to limit the scope of theinvention which is intended to cover all modifications and variations ofthis invention that come within the scope of the appended claims andtheir equivalents.

1-16. (canceled)
 17. A system for thermal ablation, comprising: aheating element heating an ablation fluid flowing through an ablationdevice and to a target region of a body; and a computing arrangementcontrolling power supplied to the heating element supplying power toheat the ablation fluid to a desired temperature, the computingarrangement reducing the supplied power when a detected ablation fluidtemperature exceeds a desired temperature and increases the suppliedpower when the detected ablation fluid temperature is below the desiredtemperature, wherein the computing arrangement monitors a percentage ofheat supplied by the heating arrangement to heat the fluid to thedesired temperature as a function of a maximum amount of heat which theheating element is capable of supplying.
 18. The system of claim 17,further comprising a thermistor coupled to the computing arrangement todetect the temperature of the ablation fluid.
 19. The system of claim17, further comprising a cooling element, wherein when the computingarrangement determines that the detected ablation fluid temperatureexceeds the desired temperature for a predetermined period of time, thecomputing arrangement provides power to the cooling element.
 20. Thesystem of claim 17, wherein when the percentage of heat holds a valuefrom −100% to +100%.
 21. The system of claim 17, wherein the powersupplied by the heating arrangement is provided with a phase delay. 22.The system of claim 17, wherein if the percentage of heat supplied bythe heating element falls below a predetermined value for apredetermined period of time, a no-flow condition is declared.
 23. Thesystem of claim 22, wherein the computing arrangement is configured toone of reduce and terminate power to the heating element when theno-flow condition is declared.
 24. The system of claim 17, wherein thecomputing arrangement supplies power to the heating element using one ofa variable phase angle control, a pulse width modulated control and anon/off control.
 25. The system of claim 22, wherein the computingarrangement is configured to provide a notification when the no-flowcondition is detected.
 26. The system of claim 25, wherein thenotification is one or more of an audible message and a visual message.27. The system of claim 22, wherein the computing arrangement isconfigured to seal a lumen of the ablation device when the no-flowcondition is detected.
 28. A method of ablating tissue, comprising:controlling a heating element to supply heat to an ablation device usinga PID algorithm, the ablation device providing a flow of ablation fluidtherethrough and into a target region of a body; regulating atemperature of fluid through the ablation device and into a targetablation region by controlling power supplied by a computing arrangementto the heating element based on an algorithm configured to supply powerto heat the ablation fluid to a desired temperature, the algorithmconfigured to reduce supplied power when detected ablation fluidtemperature exceeds the desired temperature and increase supplied powersupplied when the detected ablation fluid temperature is below thedesired temperature; and executing a safety procedure if the detectedablation fluid temperature exceeds a predetermined temperature for apredetermined period of time.
 29. The method of claim 28, wherein thesafety procedure includes sealing a lumen of the ablation device toprevent flow into the target region.
 30. The method of claim 28, whereinthe safety procedure includes supplying power to a cooling element. 31.The method of claim 28, wherein the safety procedure includes reducingpower to the heating element based on the PID algorithm.
 32. The methodof claim 31, wherein the safety procedure including temporarilyeliminating power to the heating element.
 33. The method of claim 28,wherein the safety procedure includes providing feedback to a userindicating a no-flow condition, the feedback being one or both of avisual signal and an audible signal.
 34. The method of claim 28, furthercomprising the step of monitoring a percentage of heat supplied by theheating arrangement as a function of a maximum total the heating elementis capable of applying.
 35. A system for ablating tissue, comprising: aheating element heating an ablation fluid circulating through anablation device and a target region of a body; a computing arrangementcontrolling power supplied to the heating element based on an algorithmconfigured to supply power to heat the ablation fluid to a desiredtemperature, the algorithm configured to reduce the supplied power whendetected ablation fluid temperature exceeds the desired temperature andincrease the supplied power when the detected ablation fluid temperatureis below the desired temperature wherein the computing arrangementmonitors a percentage of heat supplied by the heating arrangement toheat the fluid to the desired temperature as a function of a maximumamount of heat which the heating element is capable of supplying; and athermistor coupled to the computing arrangement to detect thetemperature of the ablation fluid.